1. Technical Field
The present invention relates in general to earth-boring drill bits and, in particular, to a bit having a combination of rolling and fixed cutters and cutting elements.
2. Description of the Related Art
The success of rotary drilling enabled the discovery of deep oil and gas reservoirs and production of enormous quantities of oil. The rotary rock bit was an important invention that made the success of rotary drilling possible. Only soft earthen formations could be penetrated commercially with the earlier drag bit and cable tool, but the two-cone rock bit, invented by Howard R. Hughes, U.S. Pat. No. 930,759, drilled the caprock at the Spindletop field, near Beaumont, Tex. with relative ease. That venerable invention, within the first decade of the last century, could drill a scant fraction of the depth and speed of the modern rotary rock bit. The original Hughes bit drilled for hours, the modern bit drills for days. Modern bits sometimes drill for thousands of feet instead of merely a few feet. Many advances have contributed to the impressive improvements in rotary rock bits.
In drilling boreholes in earthen formations using rolling-cone or rolling-cutter bits, rock bits having one, two, or three rolling cutters rotatably mounted thereon are employed. The bit is secured to the lower end of a drillstring that is rotated from the surface or by a downhole motor or turbine. The cutters mounted on the bit roll and slide upon the bottom of the borehole as the drillstring is rotated, thereby engaging and disintegrating the formation material to be removed. The rolling cutters are provided with cutting elements or teeth that are forced to penetrate and gouge the bottom of the borehole by weight from the drilistring. The cuttings from the bottom and sides of the borehole are washed away by drilling fluid that is pumped down from the surface through the hollow, rotating drillstring, and are carried in suspension in the drilling fluid to the surface.
Rolling cutter bits dominated petroleum drilling for the greater part of the 20th century. With improvements in synthetic diamond technology that occurred in the 1970s and 1980s, the fixed-cutter, or “drag” bit became popular again in the latter part of the 20th century. Modern fixed-cutter bits are often referred to as “diamond” or “PDC” (polycrystalline diamond compact) bits and are far removed from the original fixed-cutter bits of the 19th and early 20th centuries. Diamond or PDC bits carry cutting elements comprising polycrystalline diamond compact layers or “tables” formed on and bonded to a supporting substrate, conventionally of cemented tungsten carbide, the cutting elements being arranged in selected locations on blades or other structures on the bit body with the diamond tables facing generally in the direction of bit rotation. Diamond bits have the advantage of being much more aggressive and therefore drill much faster at equivalent weight-on-bit (WOB). In addition they have no moving parts, which makes their design less complex and more robust. The drilling mechanics and dynamics of diamond bits are different from those of rolling-cutter bits precisely because they are more aggressive and generate more torque. During drilling operation, diamond bits are used in a manner similar to that for rolling cutter bits, the diamond bits also being rotated against a formation being drilled under applied weight on bit to remove formation material. The diamond cutting elements are continuously engaged as they scrape material from the formation, while the rolling-cutter cutting elements indent the formation intermittently with little or no relative motion (scraping) between the cutting element and formation. Rolling-cutter and diamond bits each have particular applications for which they are more suitable than the other; neither type of bit is likely to completely supplant the other in the foreseeable future.
In the prior art, some earth-boring bits use a combination of one or more rolling cutters and one or more fixed blades. Some of these combination-type drill bits are referred to as hybrid bits. Previous designs of hybrid bits, such as is described in U.S. Pat. No. 4,343,371, to Baker, III, and U.S. Pat. No. 4,444,281 to Schumacher have equal numbers of fixed blades and rolling cutter in essentially symmetrical arrangements. In these bits, the rolling cutters to do most of the formation cutting, especially in the center of the hole or bit.
At light WOB and higher RPM, fixed-cutter or drag bits sometimes suffer from an undesirable condition known as bit whirl. In this condition, the bit rotates temporarily about an axis that does not coincide with the geometric center of the bit in such a way that the bit tends to wobble or “backwards whirl” about the borehole. Thus, individual PDC cutting elements travel sideways and backwards and are subject to high loads in a direction for which they are not designed. This can cause breakage and premature destruction of the cutting elements. Various means and methods have been devised to combat this condition in what are typically called “anti-whirl” bits. Examples of anti-whirl bits are found in commonly assigned U.S. Pat. Nos. 5,873,422 and 5,979,576 to Hansen et al. and also in U.S. Pat. No. 4,932,484, to Warren, et al., assigned to Amoco.
In rolling-cutter bits, a similar condition called “off-center running” or forward whirl occurs when the bit axis itself rotates in a concentric circle around the center of the borehole. This is typical in drilling applications in which the material being drilled is behaving plastically and lateral movement of the bit is facilitated due to lack of stabilization, light depth of cut, high RPM, and low weight on bit. Another factor encouraging off-center running of the bit is inadequate bottom hole cleaning, which leaves a layer of fine cuttings on the borehole bottom, which acts as a lubricant between the bit and the formation to make lateral displacement of the bit easier. Off-center running is not nearly as destructive to the cutting elements or cutting structure of the rolling-cutter bit as whirl is to the fixed-cutter bit. Off-center running in rolling cutter bits is still undesirable because the bit drills slowly and creates an oversize or out-of-gage borehole in which the bit is harder to stabilize and tends to “walk” so that the borehole deviates from vertical in undesirable ways. An example of a rolling-cutter design that addresses off-center running are found in commonly assigned U.S. Pat. No. 5,695,018 to Pessier and Isbell.
None of the prior art addresses the dynamic, “whirling” tendencies of the hybrid bit with its combination of rolling cutters and fixed blades. Accordingly, an improved hybrid earth-boring bit with enhanced drilling performance would be desirable.